Impact of Linking Topology on the Properties of Carbazole Trimers and Dimers

Synthesis and properties of new glass-forming 2-, 2,7-substituted carbazole dimers and trimers exhibiting ambipolar charge carrier transport are reported. The results of thorough evaluation of thermal, optical, and photoelectrical properties of the carbazole derivatives are presented. Additionally, a comparative experimental and theoretical analysis of the 2,7-substituted carbazole trimer with the 3,6-substituted analogue is performed. 2,7-Di(9-carbazolyl)-9-(2-ethylhexyl)carbazole was found to show very high electron mobility (μ e = 2 × 10 −3 cm 2 V −1 s −1 at an electric field of 1.8 × 10 5 V/cm) and reasonably high hole mobility (μ h = 8 × 10 −4 cm 2 V −1 s −1 at an electric field of 1.8 × 10 5 V/cm). 2-Substituted carbazole dimers also demonstrate bipolar conductive properties although with significantly lower charge carrier mobility values. The obtained carrier mobility values for the 2,7-substituted carbazole compound were found to be nearly 2 orders of magnitude higher than those for 3,6-substituted carbazole counterpart. The pronounced differences in carrier mobility are justified by DFT calculations indicating extended π-conjugation and a twice as large transition dipole moment in the 2,7-disubstituted carbazole derivative. This is also supported by the optical properties of the 2,7-disubstituted carbazole compound showing up to 5 times larger radiative relaxation rates, 3-fold increase in the fluorescence quantum yield and absorbance, as well as the reduced electron−vibronic system coupling as compared to the 3,6-substituted analogue. The prolonged molecular geometry and extended π-conjugation of the 2,7-substituted carbazole derivative facilitate more ordered and dense molecular packing and thus enhance intermolecular coupling in the solid films contributing to more efficient carrier hopping.